Vectors allowing amplified expression of the Saccharomyces cerevisiae Gal3p-Gal80p-Gal4p transcription switch: applications to galactose-regulated high-level production of proteins

Directed evolution of the transcription factor Gal4 for development of an improved transcriptional regulation system in Saccharomyces cerevisiae

As a well-characterized eukaryotic DNA binding transcription factor, Gal4 has been extensively employed to construct controllable gene expression systems in Saccharomyces cerevisiae. The problem of insufficient inducibility arises in the constructs with multiples genes under control of GAL promoters due to the low expression level and activity of the native Gal4. In order to obtain improved transcriptional regulation systems for multi-gene pathways, Gal4 mutants with improved activity were created by directed evolution. During the preliminary screening, five positive Gal4 variants were isolated based on the lycopene-indicated high-throughput screening method. Analysis of the mutation sites revealed that the majority of positive mutations are localized in the middle homology region with unspecified function, suggesting an important role of this domain in transactivation of Gal4. Through combinatorial site-directed mutagenesis, the best variant Gal4^T406A/V413A was obtained, which successfully increased the transcription of P_GAL-driven lycopene pathway genes and led to 48 % higher lycopene accumulation relative to the wild-type Gal4. This study demonstrates the viability of modifying Gal4 activity by directed evolution for elevated expression of P_GAL-driven genes and therefore enhanced production of the target metabolite.

Gene activation by dissociation of an inhibitor from a transcriptional activation domain

Gal4 is a prototypical eukaryotic transcriptional activator whose recruitment function is inhibited in the absence of galactose by the Gal80 protein through masking of its transcriptional activation domain (AD). A long-standing nondissociation model posits that galactose-activated Gal3 interacts with Gal4-bound Gal80 at the promoter, yielding a tripartite Gal3-Gal80-Gal4 complex with altered Gal80-Gal4 conformation to enable Gal4 AD activity. Some recent data challenge this model, whereas other recent data support the model. To address this controversy, we imaged fluorescent-protein-tagged Gal80, Gal4, and Gal3 in live cells containing a novel GAL gene array. We find that Gal80 rapidly dissociates from Gal4 in response to galactose. Importantly, this dissociation is Gal3 dependent and concurrent with Gal4-activated GAL gene expression. When galactose-triggered dissociation is followed by galactose depletion, preexisting Gal80 reassociates with Gal4, indicating that sequestration of Gal80 by Gal3 contributes to the observed Gal80-Gal4 dissociation. Moreover, the ratio of nuclear Gal80 to cytoplasmic Gal80 decreases in response to Gal80-Gal3 interaction. Taken together, these and other results provide strong support for a GAL gene switch model wherein Gal80 rapidly dissociates from Gal4 through a mechanism that involves sequestration of Gal80 by galactose-activated Gal3.

Endless versatility in the biotechnological applications of Kluyveromyces LAC genes

Most microorganisms adapted to life in milk owe their ability to thrive in this habitat to the evolution of mechanisms for the use of the most abundant sugar present on it, lactose, as a carbon source. Because of their lactose-assimilating ability, Kluyveromyces yeasts have long been used in industrial processes involved in the elimination of this sugar. The identification of the genes conferring Kluyveromyces with a system for permeabilization and intracellular hydrolysis of lactose (LAC genes), along with the current possibilities for their transfer into alternative organisms through genetic engineering, has significantly broadened the industrial profitability of lactic yeasts. This review provides an updated overview of the general properties of Kluyveromyces LAC genes, and the multiple techniques involving their biotechnological utilization. Emphasis is also made on the potential that some of the latest technologies, such as the generation of transgenics, will have for a further benefit in the use of these and related genes.

Use of a restriction endonuclease cytotoxicity assay to identify inducible GAL1 promoter variants with reduced basal activity

Inducible promoter fusions are commonly employed to study the biological functions of genes as well as to investigate mechanisms of transcription regulation. A concern for many studies of heterologous gene expression is that steady state transcription may be too high under non-inducing conditions, producing undesired phenotypes prior to induction. Fusions containing the galactose-inducible GAL1 promoter joined to PvuII, a bacterial DNA endonuclease gene, are toxic to yeast cells even under non-inducing conditions, i.e., in glucose media. This toxicity was utilized in conjunction with PCR-based mutagenesis of the GAL1 regulatory region to isolate mutant promoters that retained high inducibility but exhibited reduced basal level expression. The Mig1 repressor binding and putative TATA box regions were unchanged among four mutant promoters examined in detail. However, each promoter contained one or more mutations within previously identified binding sites for the Gal4 activator protein. Genetic assays developed to monitor GAL1p::I-SceI endonuclease-induced recombination demonstrated that basal expression from two of the new promoters (designated GAL1-V4 and GAL1-V10) was strongly reduced. These experiments and additional quantitative luciferase reporter gene assays demonstrate the utility of the approach for identifying promoters that permit more tightly controlled gene expression.

A comparative analysis of the GAL genetic switch between not-so-distant cousins: Saccharomyces cerevisiae versus Kluyveromyces lactis

Despite their close phylogenetic relationship, Kluyveromyces lactis and Saccharomyces cerevisiae have adapted their carbon utilization systems to different environments. Although they share identities in the arrangement, sequence and functionality of their GAL gene set, both yeasts have evolved important differences in the GAL genetic switch in accordance to their relative preference for the utilization of galactose as a carbon source. This review provides a comparative overview of the GAL-specific regulatory network in S. cerevisiae and K. lactis, discusses the latest models proposed to explain the transduction of the galactose signal, and describes some of the particularities that both microorganisms display in their regulatory response to different carbon sources. Emphasis is placed on the potential for improved strategies in biotechnological applications using yeasts.

A novel yeast expression system for the overproduction of quality-controlled membrane proteins

Saturation of the cell's protein folding capacity and accumulation of inactive incompletely folded protein often accompanying the overexpression of membrane proteins (MPs) presents an obstacle to their efficient purification in a functional form for structural studies. We present a novel strategy for optimization of functional MP expression in Saccharomyces cerevisiae. This approach exploits the unfolded protein response (UPR) pathway, a stress signaling mechanism that senses the accumulation of unfolded proteins in the endoplasmic reticulum. We demonstrate that a high level of UPR induction upon expression of a MP reflects impaired functional expression of that protein. Tuning the expression level of the protein so as to avoid or minimize UPR induction results in its increased functional expression. UPR status can therefore serve as a proxy variable for the extent of impaired expression of a MP that may even be applicable in the absence of knowledge of the protein's biological function.

Molecular cloning, characterisation, and expression of a neutral trehalase from the insect pathogenic fungus Metarhizium anisopliae

A neutral trehalase gene (NTH1) was isolated from a lambdaEMBL3 genomic library of the insect pathogenic fungus Metarhizium anisopliae. Sequencing of the gene revealed extensive homology with other fungal neutral trehalases. The NTH1 gene exists as a single copy in the genome. Two STREs exist in the 5'UTR of NTH1, which may mediate transcriptional activation of the NTH1 gene in response to various stresses. The NTH1 gene encodes a protein of 737 amino acids with a calculated M(r) of 83.1kDa. A cyclic adenosine 3',5'-monophosphate-dependent phosphorylation consensus site and a putative calcium binding site were found in the amino-terminal domain of NTH1, consistent with a regulatory enzyme. Expression of the trehalase cDNA was achieved in Saccharomyces cerevisiae. Southern blot analysis of RT-PCR products indicated that the neutral trehalase gene is transcribed in vitro in cell-free haemolymph of the tobacco hornworm Manduca sexta and in vivo in the early stage of infection.

Altered biology of adeno-associated virus type 2 and human papillomavirus during dual infection of natural host tissue

Adeno-associated virus (AAV), a common genital virus, may have a "protective" role against human papillomavirus (HPV)-associated cervical cancer. Epidemiological studies indicate a negative correlation between AAV infection and the incidence of cervical cancer. In contrast, HPV is positively associated with cervical cancer. To investigate interactions between these two viruses we used the organotypic "raft" culture system. The raft culture system is capable of supporting the complete HPV life cycle. Raft tissues that were actively replicating HPV were superinfected with AAV type 2 (AAV-2). We observed a multiplicity of infection (m.o.i.)-dependent enhancement and inhibition of HPV DNA replication, concomitant with AAV-2 replication. The data suggest that at low m.o.i. of AAV-2 infection, HPV DNA replication was slightly increased compared to controls and AAV-2 replicated poorly. At high AAV-2 m.o.i., HPV DNA replication was reduced and AAV-2 replicated to high levels. AAV-2 replication was increased in the presence of HPV compared to primary human keratinocyte, squamous cell carcinoma, and HaCat raft cultures infected with AAV-2 alone. These data suggest that HPV may provide types of "enhancer/helper" functions for AAV-2 replication and progeny formation. Infection with AAV-2 had significant effects on epithelial morphology. During infection with low m.o.i. of AAV-2 the epithelium stratified to a greater extent than in controls. With high m.o.i. of AAV-2 infections, tissue cytopathic effects were observed, indicating an additional factor responsible for the effect of AAV-2 on HPV replication and infection. Our results demonstrate a complex interaction between AAV-2, HPV, and skin during dual infection.